BEHAVIOUR OF STRUCTURAL STEEL UNDER CYCLIC AND NON-PROPORTIONAL LOADING

Most of the current research efforts on the inelastic behaviours of steel structures are concentrated on general uniaxial cyclic loading conditions. Very limited attention is given to the responses of structures subjected to multiaxial and nonproportional loading conditions. This is primarily because of the lacking of experimental information. The latter requires sophisticated testing equipment and data acquisition system that have become commercially available only recently.; This study is first concerned with the determination of reliable experimental information of structural steel subjected to nonproportional loading conditions. Six annealled A-36 steel tubular specimens were tested by using an MTS Axial-Torsional testing system under prescribed nonproportional strain paths. Then, the experimental data obtained were used to modify a two surface plasticity model suitable for A-36 steel with improved computational efficiency. Predictions and comparisons of the analytical model with test results of this study and those reported by others were made.; Based on this modified analytical model, cyclic moment-curvature relationships and force-deformation relationships of nonproportional bending and twisting of a thin walled structural member with circular cross section are obtained to illustrate the application of the model in the determination of the inelastic behaviours of steel structural members subjected to nonproportional loading conditions.; Finally, a pilot test program on the possible strain rate and strain rate history effect with moderate strain rate range is conducted. Some strain rate sensitivities were observed while no strain rate history effect was found.